This invention relates to an electrical junction box, and more particularly relates to an electrical junction box to be mounted on a vehicle body, such as a motor vehicle body, in which a casing body is locked on a lower cover to enhance workability of assembling the casing body and lower cover.
An electrical junction box mounted in a motor vehicle engine compartment typically includes (1) a casing body provided with a wire harness insertion opening and (2) a lower cover to be locked on the casing body. In a typical electrical junction box, the casing body is provided on an inner part of a side wall with a double wall structure having an inner wall to ensure a waterproof function. In many cases, a peripheral wall of the lower cover is inserted into a clearance in the double wall structure to couple the casing body to the lower cover.
Generally, in a waterproof electrical junction box, the peripheral wall of the lower cover and the double wall structure of the casing body are partially inclined. This requires that a worker pay particular attention to the opposite ends of the slant portions of the lower cover and the double wall structure. Furthermore, if there are additional fitting portions on the lower cover and casing body, the assembling work will become difficult and burdensome to the worker because the worker must take care of the additional fitting portions as well as the regular fitting portions. Accordingly, it is desirable to enhance workability of assembling the casing body and lower cover while maintaining the waterproof function by improving the fitting structure of the casing body and lower cover.
A fitting structure for a casing body and a lower cover, which restricts water from entering an interior of an electrical junction box, has been proposed. For example, FIG. 9 shows an electrical junction box disclosed in JP 2005-204356A. The electrical junction box includes a main cover 1 provided on a side wall 1a with an engaging protrusion 1b and an upper cover 2 provided with an engaging frame portion 2a opposed to the engaging protrusion 1b. The engaging protrusion 1b is coupled to the engaging frame portion 2a so that the upper cover 2 can be rotatably attached to the main cover 1. The main cover 1 is provided on a side wall with an opening 1c for a terminal. A cover wall 2b is formed on the upper cover 2 to close the opening 1c. 
When the engaging protrusion 1b of the main cover 1 is coupled to the engaging frame portion 2a of the upper cover 2 and when the upper cover 2 is mounted on the main cover 1, (1) the cover wall 2b of the upper cover 2 closes the opening 1c in the main cover 1 and (2) a distal end of the cover wall 2b is inserted into a guide engagement portion 1d of the main cover 1. Since the guide engagement portion 1d pushes the cover wall 2b inward, it is possible to restrict water from entering a clearance between the side wall 1a of the main cover 1 and the cover wall 2b of the upper cover 2.
However, during high pressure water washing, the side wall 1a of the main cover 1 or the cover wall 2b of the upper cover 2 may be subject to a deflection. This creates a possibility of water entering from the opposite sides of the cover wall 2b into the opening 1c, since the side wall 1a of the main cover 1 merely contacts the cover wall 2b of the upper cover 2. Consequently, the electrical junction box has an inadequate waterproof function.
Additionally, when the upper cover 2 is mounted on the main cover 1, the engaging protrusion 1b of the main cover 1 may become disengaged from the engaging frame portion 2a of the upper cover 2. Furthermore, since a worker must insert a distal end of the cover wall 2b into the guide engagement portion 1d while the worker also takes care of engagement between the engaging protrusion 1b and the engaging frame portion 2a, the work will become difficult and workability will deteriorate.